The present invention relates to a fuse used to protect electronic parts and equipment from an overcurrent.
A fuse generally performs its function of protecting circuit components from overcurrent by breaking the circuit when a fuse element, generally a metal wire, is heated by Joule heat to its melting point.
When the fuse element melts, the metal vaporizes, causing the pressure within an envelope containing the fuse element, to increase. This increase in internal pressure can substantially damage the fuse.
The protecting role of a fuse is completed when the smallest part at the center of the fusible wire, the part where heat dissipation is most intense, melts. However, the current that flows into the metal wire is large. Thus the entire wire melts, often vaporizing instantly.
Hence fusion caused by the overcurrent instantly generates a large amount of thermal energy, thereby increasing the pressure within the envelope containing the fuse element. Such an increase in internal pressure places a mechanical load on the envelope. In the worst case, the increase in internal pressure causes damage to or breakage of the envelope containing the fuse element.
To ensure reliable functioning, some conventional fuses provide extra space within the envelope containing the fuse element and position terminals connected to the fuse element outside the envelope. The dimensions of the extra space are chosen in relation to the amount of metal in the fuse element.
The increasing miniaturization of electronic circuitry makes strong demands for compact fuses. It is difficult to make a compact fuse with the configuration just described, which ensures the circuit-breaking function of a fuse and prevents damage to the envelope containing the fuse element merely by providing extra space therein.
To overcome this problem, prior art fuses have covered the fusible metal wires with flexible synthetic resin, thereby substituting the resin for the extra space. Such a fuse is disclosed, for example, in Japanese Utility Model Publication No. 38988/1983. However, covering fusible metal wires with flexible synthetic resin presents a problem. Such a configuration cannot ensure sufficient circuit-breaking function while simultaneously resisting mechanical load from the envelope containing the fuse element.